Three-dimensional, position-sensitive radiation detection

ABSTRACT

Disclosed herein is a method of determining a characteristic of radiation detected by a radiation detector via a multiple-pixel event having a plurality of radiation interactions. The method includes determining a cathode-to-anode signal ratio for a selected interaction of the plurality of radiation interactions based on electron drift time data for the selected interaction, and determining the radiation characteristic for the multiple-pixel event based on both the cathode-to-anode signal ratio and the electron drift time data. In some embodiments, the method further includes determining a correction factor for the radiation characteristic based on an interaction depth of the plurality of radiation interactions, a lateral distance between the selected interaction and a further interaction of the plurality of radiation interactions, and the lateral positioning of the plurality of radiation interactions.

This invention was made with government support under Contract No.:DE-FG03-01NN20122 awarded by Department of Energy. The government hascertain rights in the invention.

BACKGROUND OF THE DISCLOSURE

1. Field of the Disclosure

The disclosure relates generally to radiation detection and, moreparticularly, to three-dimensional, ionization-based radiationdetection.

2. Brief Description of Related Technology

In view of the variety of ways in which radiation is generated andencountered, radiation detectors have been used to determine a number ofdifferent characteristics of the radiation, including radiation type,energy, source (e.g., the isotopes emitting the radiation), sourceintensity, and source location. The two main types of radiation—neutralparticles such as photons (x-rays and gamma rays) and neutrons, andcharged particles such as fast moving electrons and protons—havegenerally been detected with three types of radiation detectors, namelygas, scintillation and semiconductor detectors.

When gamma rays interact with a detector medium, charge carriers (e.g.,electrons) are generated via electron ionization. The initial kineticenergy of the electrons equals the energy loss of the gamma ray.Furthermore, the number of created charge carriers is proportional tothe energy deposition of each interaction. Both negative and positivecharge carriers, such as electrons and holes in a semiconductor device,then move toward, and are eventually collected by, an anode (apositively biased electrode) and a cathode (a negatively biasedelectrode), respectively. The induced signals on the electrodes areproportional to the number of charge carriers. As a result, the amountof energy deposition has generally been determined by measuring theamplitude of the induced signal on an electrode.

Semiconductor detectors are favorable in gamma ray detection for theirhigh atomic number, high density and low ionization energy forgenerating each free-moving charge carrier. Unfortunately, eachcurrently available semiconductor medium presents limitations. Silicondetectors, for instance, have fairly low atomic number and the typicalthickness is only a few millimeters. With the resulting low detectionefficiency for gamma rays, silicon detectors are normally used to detectx-rays and charged particles. Other options include high-puritygermanium detectors, which present a modest atomic number and gooddensity, and can be produced in large sensitive volumes. While achievingexcellent energy resolution and high detection efficiency for gammarays, germanium detectors unfortunately require operation at liquidnitrogen temperatures to avoid spurious signals arising from a smallband-gap energy.

Wide band-gap semiconductor materials, especially CdZnTe (CZT), havepotential for both good energy resolution and compatibility withroom-temperature operation. However, a number of challenges arepresented by these room-temperature semiconductor detectors. Holes inCdZnTe move very slowly and are easily trapped, and thus contributelittle, if at all, to the induced signal. As a result, the inducedsignal is mainly contributed by the movement of electrons, which, inturn, makes the signal amplitude dependent on the drift length of theelectrons. Even for the same energy deposition, the induced signal thenhas a different amplitude depending on where the electrons are created.Various methods have been proposed and evaluated to overcome thisproblem, such as pulse shape discrimination, pulse compensation, andsingle-polarity charge sensing techniques. Unfortunately, the electronscan also be trapped during their drift to the anode, causing a deficitin the induced signal amplitude. Techniques using an optimized relativegain between two anodes and depth sensing have been proposed to addressthis electron trapping effect. Unfortunately, the energy resolution hasremained far worse than the theoretical limit due to materialnon-uniformity.

Single-polarity charge sensing techniques, such as coplanar orpixellated anodes, have been utilized to minimize the hole-trappingproblem and improve the energy resolution for larger volume detectors.Unfortunately, these techniques were still limited by problems arisingfrom material non-uniformity and spatially varying electron trapping,thereby limiting the energy resolution of, for instance, co-planar griddetectors.

More recently, the foregoing challenges were addressed in thedevelopment of three-dimensional CZT spectrometers by He et al. and Liet al. See, for example, Z. He, et al., “3-D position sensitive CdZnTegamma-ray spectrometers,” Nucl. Instrum. Meth. A, vol. 422, pp. 173-178(1999) and W. Li, et al., “A data acquisition and processing system for3-D position sensitive CZT gamma-ray spectrometers,” IEEE Trans. Nucl.Sci., vol. 46, pp. 1989-1994 (1999). By determining thethree-dimensional (3-D) position information for a single-pixel event,the material non-uniformity and varying electron trapping effects wereaddressed. Unfortunately, these devices were incapable of correctlydetermining the information for multiple-pixel events.

SUMMARY OF THE DISCLOSURE

In accordance with one aspect of the disclosure, a method is useful fordetermining a characteristic of radiation detected by a radiationdetector via a multiple-pixel event having a plurality of radiationinteractions. The method includes determining a cathode-to-anode signalratio for a selected interaction of the plurality of radiationinteractions based on electron drift time data for the selectedinteraction, and determining the radiation characteristic for themultiple-pixel event based on the cathode-to-anode signal ratio.

In some cases, the method further includes determining the electrondrift time data for the selected interaction, such that the radiationcharacteristic is determined based on both the cathode-to-anode signalratio and the electron drift time data.

The radiation characteristic may correspond with an interaction depthwithin the radiation detector of the selected interaction.Alternatively, the radiation characteristic corresponds with an energydeposited by the radiation. The radiation characteristic determinationmay then include correcting data for the energy utilizing a correctioncoefficient based on the cathode-to-anode signal ratio.

In some embodiments, the cathode-to-anode signal ratio determinationincludes correlating the electron drift time data with thecathode-to-anode signal ratio. The correlation may then include the stepof accessing a calibration look-up table correlating electron drifttimes with corresponding cathode-to-anode signal ratios. Thecorresponding cathode-to-anode signal ratios may be based on empiricaldata from single-pixel interaction events, and the calibration look-uptable may specify a number of energy correction factors correspondingwith the cathode-to-anode signal ratios. The energy correction factorsmay be anode pixel-specific.

In some cases, the method further includes adjusting the radiationcharacteristic to correct for a crosstalk effect between the pluralityof radiation interactions. The correction factor determination mayinclude combining multiple correction factors corresponding withmultiple respective pixel pairs of the radiation detector with which theradiation interacts in the multiple-pixel event.

In some embodiments, the method further includes determining acorrection factor for the radiation characteristic based on a centroiddepth of the plurality of radiation interactions.

Alternatively or in addition, the method further includes determining acorrection factor for the radiation characteristic based on a distancebetween a pair of pixels of the radiation detector with which theradiation interacts in the multiple-pixel event.

Alternatively or in addition, the method further includes determining acorrection factor for the radiation characteristic based on lateralpositioning of a pair of pixels of the radiation detector with which theradiation interacts in the multiple-pixel event. The correction factordetermination may include determining whether one or both of the pair ofpixels are disposed along an anode periphery.

In an alternative embodiments, the method further includes determining acorrection factor for the determined radiation characteristic based onan interaction depth of the plurality of radiation interactions, and alateral distance between the selected interaction and a furtherinteraction of the plurality of radiation interactions. The correctionfactor may be further based on a lateral position of the selectedinteraction.

In accordance with another aspect of the disclosure, a radiationdetector is useful for detecting a multiple-pixel event having aplurality of radiation interactions. The radiation detector includes afirst data processing module that generates electron drift time datafrom anode and cathode signals arising from the plurality of radiationinteractions, and a second data processing module that accesses a firstdata correlation of electron drift times with cathode-to-anode signalratios and a second data correlation of cathode-to-anode signal ratioswith correction factor values for a radiation characteristic for themultiple-pixel event. The second data processing module determines theradiation characteristic based on the electron drift time data, thefirst data correlation and the second data correlation.

In some cases, the second data processing module also determines aninteraction depth for a selected radiation interaction of the pluralityof radiation interactions based on the electron drift time data and thefirst data correlation. The second data processing system may furtherdetermine a distance between a first pixel of the radiation detectorassociated with the selected radiation interaction and a second pixelwith which the radiation also interacts in the multiple-pixel event. Thesecond data processing system may then also access a third datacorrelation to determine a further correction factor for the radiationcharacteristic based on the interaction depth and the distance betweenthe first and second pixels. The third data correlation may specifyvalues for the further correction factor based on lateral positioning ofthe first and second pixels. The values for the further correctionfactor may vary based on whether one or both of the first and secondpixels are disposed along an anode periphery or within a central anodearea.

In accordance with yet another aspect of the disclosure, a method ofcalibrating a radiation detector using a radiation source having a knownenergy level includes determining a lateral distance between anodepixels of the radiation detector involved in a multiple-pixel eventarising from radiation from the radiation source interacting with theradiation detector, calculating a multiple-pixel event correction factorvalue based on the known energy level of the radiation source, andstoring the calculated correction factor value in association with thelateral distance for the multiple-pixel event.

In some cases, the method further includes determining respectiveinteraction depths for first and second interactions of themultiple-pixel event, and storing the calculated correction factor andthe lateral distance in association with the respective interactiondepths. The method may then further include determining lateral positiondata for the first and second interactions such that the calculatedcorrection factor is further stored in association with the lateralposition data.

Alternatively or in addition, the method further includes determining acathode-to-anode signal ratio for a single-pixel event arising from theradiation from the radiation source interacting with the radiationdetector, determining an electron drift time for the single-pixel event,and storing a correlation of the electron drift time data and therespective anode-to-cathode signal ratio data. The storing step mayinclude storing a radiation energy correction factor for thesingle-pixel event in connection with the correlation.

In some embodiments, the method further includes determining a centroiddepth for the multiple-pixel event and storing the calculated correctionfactor value in association with the centroid depth.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

For a more complete understanding of the disclosure, reference should bemade to the following detailed description and accompanying drawingfigures, in which like reference numerals identify like elements in thefigures, and in which:

FIG. 1 is a schematic representation of a radiation detector inaccordance with one aspect of the disclosure;

FIG. 2 is a schematic representation of signal readout circuitry of theradiation detector of FIG. 1 in accordance with one embodiment;

FIG. 3 is a schematic representation of a multiple-pixel event having aplurality of radiation interactions with a semiconductor crystal of theradiation detector of FIG. 1;

FIG. 4 is a schematic representation of the semiconductor crystal ofFIG. 3 showing a multiple-pixel anode configuration and a volumetricdivision of the crystal for three-dimensional position sensing;

FIG. 5 is a schematic representation of a cross-sectional, partial viewof the radiation detector of FIG. 1 in connection with the detection ofa single-pixel event;

FIG. 6 is a plot of normalized signals induced or generated inconnection with the detection of the single-pixel event depicted in FIG.5;

FIG. 7 is a schematic representation of a cross-sectional, partial viewof the radiation detector of FIG. 1 in connection with the detection ofa multiple-pixel event;

FIG. 8 is a multi-variable plot of signals induced or generated inconnection with the detection of the multiple-pixel event depicted inFIG. 7;

FIG. 9 is a plot of exemplary empirical data correlating electron drifttimes with cathode-to-anode signal ratios arising from single-pixelevents detected by an exemplary radiation detector and for use inconnection with one aspect of the disclosure;

FIG. 10 is a block diagram of a radiation detection technique formultiple-pixel events in accordance with one aspect of the disclosure;

FIG. 11 is a block diagram of a radiation detection technique formultiple-pixel events in accordance with another aspect of thedisclosure;

FIG. 12 is a block diagram of a radiation detector calibration techniquefor support of the detection technique of FIG. 10 and in accordance withone aspect of the disclosure;

FIG. 13 is a block diagram of another radiation detector calibrationtechnique for support of the detection technique of FIG. 11 and inaccordance with another aspect of the disclosure; and,

FIGS. 14A-14C are energy spectra plots generated by an exemplarydetector for single-pixel events, two-pixel events and three-pixelevents, respectively.

While the disclosed system and method are susceptible of embodiments invarious forms, there are illustrated in the drawing (and will hereafterbe described) specific embodiments of the invention, with theunderstanding that the disclosure is intended to be illustrative, and isnot intended to limit the invention to the specific embodimentsdescribed and illustrated herein.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Described below are three-dimensional radiation spectroscopy techniquesfor semiconductor-based detectors capable of sensing multiple-pixelevents. The disclosed techniques include aspects of both detectorcalibration and the subsequent data acquisition enabled thereby.Generally speaking, three-dimensional radiation detection in amultiple-pixel event context is supported by the disclosed techniquesvia one or more correction factors for the calculation of acharacteristic of the radiation, such the deposited energy. For example,a first correction factor may be based on a correlation ofcathode-to-anode signal ratios and electron drift times. A secondcorrection factor may be based on one or more of the following: theinteraction depth, the relative lateral positioning of the interaction,and the lateral distance between interactions. These correction factorsare generated as a result of the calibration techniques disclosed hereinand, taken together or individually, support the improved resolution ofthe disclosed radiation detector.

A software platform to implement the disclosed calibration and dataacquisition techniques is disclosed herein in conjunction with aphysical hardware platform, including circuitry and other devices andequipment for radiation detection via a wide band-gap semiconductormedium. Although described in connection these software and hardwareplatforms, the disclosed techniques are not limited to practice orimplementation with the exemplary hardware and software platforms andarchitectures described below. For example, the disclosed techniques arealso suitable for use in other ionization detection contexts, such asthose utilizing gas detectors. Moreover, practice of the disclosedtechniques is not limited to any one semiconductor material. Stillfurther, the disclosed techniques may be implemented in a variety ofradiation detection applications and contexts, and are not limited tothe radiation sources described herein.

With reference now to FIG. 1, an exemplary embodiment of a radiationdetector system indicated generally at 20 includes a front-end board 22having mounted thereon a number of readout electronics circuits (orchipsets) 24 and a detector module 26 having a wide band-gapsemiconductor detector medium. In this exemplary case, the detectormodule 26 may include a CdZnTe detector mounted on a ceramic plate.Practice of the disclosed technique is not limited to any specificsemiconductor material, but suitable crystals may be obtained fromeV-PRODUCTS (Saxonburg, Pa.). Several exemplary detector module 26, theresults of testing of which are discussed below, utilizes crystals withapproximate dimensions of 1.5×1.5×1 cm³.

As described below, the detector module 26 has a pixellated anodehaving, for instance, an 11×11 pattern that establishes 121 anodepixels. In this case, the signals from the 121 anode pixels and thecathode are read out by the four readout chipsets 24. In one embodiment,the pixellated anodes of the module 24 are wire-bonded to the readoutelectronics using an intermediate ceramic substrate (not shown) withplate-through-via(s). The chipsets 24 may be configured in any one of anumber of ways known to those skilled in the art, including, forexample, as an application-specific integrated circuit (ASIC).

Generally speaking, each chipset 24 has a number of channels used toread out the induced charges on the anode pixels, as well as thecathode. In this specific example, each chipset 24 includes a first ASIC28 dedicated to timing data acquisition and a second ASIC 30 dedicatedto energy data acquisition. Each ASIC 28, 30 includes a number ofchannels (e.g., 32) for acquiring and capturing data indicative of theradiation interaction events occurring in the detector medium 26. Morespecifically, the ASIC 28 has a number of channels used to trigger thesystem 20 and read out electron drift times and, accordingly, may bereferred to herein as the Trigger ASIC with Timing, or TAT, ASIC.Similarly, the ASIC 30 has a number of channels to read out signalsindicative of the charges induced on the anode pixels and, accordingly,may be referred to herein as the voltage ASIC with stretching, or VAS,ASIC. The two ASICs 28, 30 may be coupled to support communicationtherebetween in a number of ways. For example, a preamplifier output ofeach VAS channel may be wire-bonded to an input of a corresponding TATchannel.

More generally, connections between the components mounted on thefront-end board 22 may vary based on the system architecture andcomponents, as well as take into consideration factors such as noisesuppression. The nature of the connections may vary, for instance, basedon the degree to which components of the system 20 are integrated intoone chip, board, card, etc. In this exemplary case, conducting traceswithin the multi-layer ceramic plate connect every anode pixel to acorresponding metal pad (not shown) on the periphery of the plate. Ashort wire-bond (not shown) then connects each pad on the ceramic plateto the input of each ASIC channel on the front-end board 22.

A controller and repeater card 32 is used to generate and send readoutclock signals to the ASICs 28, 30 and also convert the output of eachASIC 28, 30 to a voltage signal for input to a data acquisition (DAQ)board 34, such as the PCI-6110 DAQ board from National Instruments. TheDAQ board 34 may provide an A/D converter 36 and a controller interface38 for further data processing elements 40 of the detector system 20.The data processing elements 40 may implement one or more aspects of thedisclosed techniques in conjunction with one or more calibrationdatabases or data sets 42 stored in one or more memories. In some cases,one or more of the calibration data sets may be represented by, orprovided via, a fitted or other function, the parameters of which may bestored in the same or different memories of the system 20.

More generally, the data processing elements 40 may include hardware,software, or firmware components, or any combination thereof. In oneexemplary case, the data processing steps are coded in C++, and providefor automatic control and implementation of the disclosed techniques.

In some embodiments, the chipsets 24 may include or incorporate thefunctionality of one or more of the following: peak-hold circuits,triggering circuits with masking, time sensing capability andAC-coupling equivalent circuits. The time-sensing functionality andother signal processing functions provided by the chipsets 24 aredescribed further below.

In operation, each anode pixel location provides the lateral coordinatesof a radiation interaction, while cathode/anode signals and electrondrift time data are both acquired and used to obtain the interactiondepth. As described below, data-acquisition and processing software inaccordance with the disclosed techniques may then implement eithercalibration and, more generally, real-time spectroscopy. The calibrationprocedures may be completed automatically by the software or manually,as desired. The final results of the calibration procedures may then befed back to the data acquisition program(s) to support implementation asa real-time 3-D CZT spectrometer with COMPTON imaging capabilities.

With reference now to FIG. 2, further details regarding the ASICs 28, 30are shown in connection with exemplary VAS and TAT channels indicatedgenerally at 44 and 46, respectively, and separated for convenience inillustration by dashed line 48. In this exemplary embodiment, eachindependent channel 44 of the VAS ASIC 30 includes a preamplifier 50, a1-μs time shaping amplifier 52, a peak-hold circuit 54 and a sample-holdcircuit 56. The first channel on each VAS chip 30 may have an oppositepolarity to the other 31 or 32 channels, in order to read out the signalfrom the cathode.

Each TAT channel includes a 75-ns-shaping time fast shaper 58, adiscriminator 60 for triggering, and a time-to-amplitude converter (TAC)62 for electron drift time sensing. In some cases, a trigger mask may beset to disable those channels having high noise, if desired.

In operation, the ASICs 28, 30 may reside in two readout modes: asingle-channel mode and a serial readout mode. The single-channel modemay be used for system testing in which a multiplexer is set so that onechannel is constantly connected to the output and a test pulse generatedby the DAQ board 34 is injected into the respective channel. The outputof the channel is then read out continuously such that the response ofthe channel, including voltage and timing responses, to the test pulsecan be monitored both in a DAQ program and on an oscilloscope. Theserial read out mode may be used to sequentially sample the output ofall the channels by the multiplexer being automatically switched channelby channel by the readout clock.

The foregoing detector electronics generally support the analysis ofmultiple-pixel events in which a plurality of radiation interactionsoccur inside the detector module 26. FIG. 3 depicts a generic detectorvolume 64 to illustrate a multiple-pixel event for which one or moreradiation characteristics are determined in accordance with thedisclosed techniques. When interactions occur inside the detectorvolume, energies E₁, E₂ . . . are deposited at respective positions(x₁,y₁,z₁),(x₂,y₂,z₂) . . . and, generally speaking, the disclosedtechniques provide a method to determine data values for these energiesand positions for all of interactions of the multiple-pixel event, i.e.,(E_(i),x_(i),y_(i),z_(i)), with high resolution. To this end, detectorsgenerally use the charge carriers generated via the ionization processesof each radiation interaction as the mechanism of determining thedeposited energies and their positions. The number of charge carriersN_(i) (electron-ion pairs for gaseous detectors, or electron-hole pairsfor semiconductor detectors) is proportional to the energy depositionE_(i). The radiation detection system 20 generally measures the inducedcharges on the electrodes to deduce N_(i) and also ( x _(i), y _(i), z_(i))—the centroid position of the charge carriers of the i-thinteraction. The centroid position ( x _(i), y _(i), z _(i)) may beregarded as a good approximation of the actual position of i-thinteraction (x_(i),y_(i),z_(i)) and is also determinative of theposition resolution.

With reference now to FIG. 4, and as set forth above, the detectormodule 26 uses a pixellated anode pattern 66 (e.g., 11×11) and aconventional planar cathode disposed on an opposing side (not shown).The anode pixel pitch in one exemplary embodiment is 1.27 mm. Betweenpixel anodes 68, the pattern 66 includes a common grid 70 biased atnegative voltage to focus the electrons to the pixel anodes 68. Thetrace width of the grid electrode may be, for instance, about 100 μmwith a gap of about 200 μm between the grid 70 and the pixel 68.

Employing a pixellated anode electrode provides a way of reading outboth the amplitude of the induced charge signal and the (x,y)coordinates for each specific interaction. The z coordinate may beobtained by measuring how long it takes the electrons to drift fromwhere they are generated to where they are collected at the anode pixel68. Generally speaking, pulse waveform analysis may be applied to boththe pixel anode signal and the cathode signal to get the depositedenergy and the interaction depth z. By choosing proper time-shapingand/or other signal processing techniques, the cathode signal may bemade sensitive to both the number of electrons generated in theionization process and the drift length of the electrons beforecollection by the anode pixel 68. Meanwhile, the anode signal is onlysensitive to the number of electrons generated. Therefore, the ratio ofthe cathode signal to the anode signal (or other forms of functions ofthe two signals) may provide the interaction depth z. Based on theforegoing and the procedures described below, the disclosed techniqueaddresses the correction of variation in detector response due toelectron trapping, material non-uniformity and weighting potentialvariations.

The parameters underlying the computation or acquisition of theaforementioned radiation characteristics are now described in connectionwith the detection of single-pixel events. Based on that discussion, theparameters and computations may then be extended and applied to themultiple-pixel event detection context.

A detector generally has X×Y×Z dimensions, a planar cathode and N×Mpixellated anodes. If the expected and achievable depth resolution isΔz, the whole active detector volume may be virtually divided into N×M×Dvoxels, where

$D = {\frac{Z}{\Delta\; z}.}$As schematically shown in FIG. 4, each voxel 72 has a very small volume(compared to the total detection volume) such that the detector responsechanges very little within each of the voxels 72. It follows that theaverage detector response in each voxel 72 may be used as the actualdetector response within the whole voxel 72. While the dimension of thevoxel 72 and the variation of detector response inside each voxel 72delimit the achievable position resolution and energy resolution of thisdetector, practice of the disclosed techniques is not limited to anyparticular voxel size or dimension.

By using one or more calibrations for every pixel (x,y), the detectorresponse coefficients A(x,y,z) for all depths of interaction may befound that satisfy the relation Q(x,y,z)=A(x,y,z)E where Q(x,y,z) is theinduced charge signal on the pixel electrode at (x,y) when asingle-interaction event occurs and deposits energy E under pixel (x,y)at depth z. The output signal from the readout electronics 24 (FIG. 1),S(x,y), should have a monotonic relationship (a linear relationship istypical) to Q(x,y). Thus, the measurement on S(x,y) may be used todeduce the corresponding Q(x, y). This monotonic relationship is givenby the readout electronics response, which may be calibrated using oneor more calibration data sets. The calibration procedures to generatethe data sets generally apply a source of known energy E₀ to thedetector volume followed by the measurement of the electronic outputsignal S(x,y). Using the relationship between S(x,y) and the inducedcharge on each electrode Q(x,y), one may obtain the relationship betweenQ(x,y,z) and the known energy E_(o), and thus the system responsecoefficients A(x,y,z).

Based on the foregoing generalized analysis, each calibration step orprocedure is useful as an event reconstruction mechanism, in the sensethat the original information carried by the radiation and conveyed viathe detector interaction (e.g., deposited energy, position, etc.) mustproceed all the way through the radiation ionization process in thedetector medium, the charge carrier transportation in the detector, andthe charge induction process, and then finally be processed by thereadout electronics. The information will naturally be distorted viathese processes, due to, for instance, random nature of chargegeneration, position dependence on charge trapping and induction, anddiscretization of, and variations in, the readout electronics, such asbaseline offset, gain drift and non-linearity.

In order to reconstruct the original information of each radiationinteraction, one or more corrections (or correction factors) areintroduced to address the aforementioned distortions introduced into thesignal chain. One or more of the correction factors may be specified orgenerated via the disclosed calibration techniques. With thesetechniques, the limitations of the detector system 20 may be removed toreveal the most accurate representation of the information possiblegiven the energy resolution and the position resolution of theinteraction. As described above, the corrections may be affected viafitted or other functions the parameters of which may have beenspecified as a result of the disclosed calibration techniques.Accordingly, the determination of characteristics of the radiation andany one or more of the correction factors may be implemented via datalook-up tables or, in other cases, one or more functions derived fromthe calibration techniques disclosed herein.

The readout electronics generally provides two groups of signals—theamplitude and timing signals. Therefore, the calibration of the readoutsystem has two corresponding categories. The first calibration categoryinvolves a determination of the relationship between the input chargepulse amplitude Q and the output signal amplitude S. The calibration ofthe signal amplitude normally includes the calibration on baselineoffset, gain and linearity, and their drift under different conditionssuch as count rate, time and temperature. The goal of these calibrationsis to find out the true input signal corresponding to the recordedoutput signal. The second calibration category involves the relationshipbetween the input pulse timing t and the output timing signal T. Thecalibration of the timing signal normally includes the calibration onthe time-to-amplitude gain and the time-amplitude-walk. If leading edgetriggering circuitry is used for the electron drift time determination,the problem of time-amplitude walk may be corrected via techniques knownto those skilled in the art. In some cases, in order to get the truetiming signal for any pulse amplitude, the time-amplitude walk should becalibrated. Although there are multiple ways in which to address thiscalibration, further information is set forth below in connection withthe exemplary embodiment depicted in FIG. 8. Practice of the disclosedtechniques, however, is not limited to any one way or mechanism ofaddressing the time-amplitude walk.

In general, calibration directed to the signal amplitudes may proceedusing a single, known gamma-ray energy to represent the detectorresponse for the entire energy range of interest. A gamma-ray sourcewith a single and high-energy emission may be used to create a spectrumin which the photopeak is more easily located. The high energy allowsthe gamma ray to penetrate deep into the detector to ensure that thedetector response at all depths is calibrated. For example, thecalibration techniques disclosed herein may conveniently use ¹³⁷Cs as acalibration source in view of its simple 662 keV photopeak. However,other sources may be used for the calibration such that implementationof the disclosed system and method is not limited to any one particularsource.

The calibration techniques disclosed herein generally address twoeffects presented by multiple-pixel events, namely (i) positiondependence due to charge trapping and variation of the weightingpotential, and (ii) the signal deficit arising from weighting potentialcross-talk between multiple interactions.

The manner in which radiation characteristics are determined insingle-pixel events is now described. The calculation of the interactiondepth and the parameter relationships leading thereto may then beutilized in the multiple-pixel event context.

FIG. 5 shows the technique of interaction depth sensing using the ratiobetween the cathode signal and the anode pixel signal when only oneinteraction occurs in the detector. For an energy deposition arisingfrom an interaction of a gamma ray in the detector at depth z, both theanode signal V_(a) and the cathode signal V_(c) are read out. Because ofthe single-polarity charge sensing, V_(a) should be approximatelyproportional to the number of electrons generated by the interaction.Because the cathode is a conventional planar electrode, V_(c) should beproportional to the product of the interaction depth and the number ofelectrons. Therefore, the ratio of V_(c) to V_(a) will be proportionalto the interaction depth z.

FIG. 6 illustrates the nature of the single-polarity charge sensingtechnique underlying this determination. More specifically, FIG. 6 showsthe difference in the induced charge on the anode pixel and the cathodeas a function of depth, while illustrating the linear depth dependenceof the cathode signal and the weak depth dependence of the anode pixelsignal.

Turning to the multiple-pixel event context, FIG. 7 depicts a pair ofgamma ray interactions within the detector volume at depths z₁ and z₂.For these multiple interactions, the cathode signal is contributed bymultiple energy depositions and thus cannot be used alone to obtain theinteraction depth for each individual interaction. Fortunately, becausethe electrons drift at a nearly constant velocity in the detector,electron drift times may be determined and recorded to enable thedetermination of the interaction depth information for all interactionsof the event.

One way in which the electron drift time may be measured or determinedis now described. Practice of the disclosed techniques, however, is notlimited to any one drift time determination or calculation methodology.When a gamma ray interacts inside the detector and the electron cloudsstart to drift, a cathode trigger is generated when the induced signalon the cathode crosses a threshold. When an electron cloud arrives inthe vicinity of an anode pixel, the induced signal crosses a threshold,and triggers the corresponding anode pixel. The individual electroncloud drift times may then be derived from the time difference betweenthe cathode trigger and the anode triggers.

The multivariate plot of FIG. 8 illustrates the timing sequences ofthese electron drift time measurements for multiple-pixel events.Starting at the top level of the plot, a trigger pulse 80 is induced(i.e., generated) by the TAT channel for the cathode when a gamma-rayinteracts in the detector. The electron clouds start to drift, and theinduced signal on the cathode crosses a threshold. The trigger pulse 80starts the TAC 62 (FIG. 2) in the TAT cathode channel and generates asystem trigger (i.e., associated with the HOLD pulse shown at the bottomof the plot). When an electron cloud eventually drifts sufficiently nearan anode pixel, the induced signal crosses a threshold, and triggers thecorresponding TAT channel with a pulse 82. This trigger pulse 82 startsthe TAC corresponding to that anode pixel channel. One or moreadditional anode pixels may also be triggered (see, e.g., pulse 84).After a fixed delay after the system trigger, all the channels are readout in serial mode through a multiplexer built into the chipsets 28, 30.By using peak-hold in addition to sample-hold circuitry, the pulseamplitude of the multiple-pixel events with different electron drifttimes (i.e., different peaking times) may be read. The individualelectron cloud drift times may then be derived from the timing signalgenerated by the TACs in the TAT channels (i.e., as the time differencebetween the cathode trigger and the anode trigger).

As described above, for single-pixel events, the ratio of the cathodesignal and the anode signal may be used to get the interaction depth.This depth information may then be used to sort the events into spectrafor different depths. Following that, the spectra from all the depthsmay be combined by aligning the photopeak (e.g., 662 keV) to the sameposition. As a result, the material non-uniformity and the variations inelectron trapping and weighting potential are corrected down to thelimit of the position resolution over the whole detector volume. Formultiple-pixel events, although we can only get the interaction depthfrom the electron drift time, depth correction coefficients derived fromthe C/A ratio in the single-pixel events calibration are used asdescribed below. Thus, the relation between the C/A ratio and theelectron drift time is first determined for single-pixel events viaempirical data such as that shown in FIG. 9. Then, for multiple-pixelevents, the equivalent C/A ratio is deduced from the electron drift timefor each pixel. As a result, mapping the electron drift time to the C/Aratio and using the depth correction coefficients derived from the C/Aratio result in better energy resolution than only using the electrondrift time.

The steps taken to apply the depth correction factor are shown inaccordance with one aspect of the disclosure in FIG. 10. After amultiple-pixel event is detected in a block 90, the above-described orsimilar signal processing may be implemented to determine the electrondrift time data for each interaction in a block 92. A block 94 thendetermines the above-described correlation between the drift times andthe C/A ratio by, for instance, accessing a look-up table, database ordata set (see, e.g., the calibration database 42 shown in FIG. 1). Usingthe correction factor, a block 96 may then determine the energy of theinteraction, or some other radiation characteristic, with improvedresolution. Alternatively, or in addition, the correlation may be usedto determine the interaction depth, as discussed above based on the C/Aratio. To that end, another look-up table, database or data set may beaccessed to determine the interaction depth corresponding with the C/Aratio determined from the drift time data.

In some embodiments, the correction factors may be specified on apixel-specific basis. In this way, material non-uniformities may beaddressed. Alternatively, the database or data set may incorporateapproximations, averaging or other data processing to minimize thestorage size of the data set, or for any other desired purpose (e.g.,calculation speed for real-time processing).

There may be one or more additional corrections supported by calibrationof the radiation detector. Two examples are corrections for baselinedrift and gain drift.

Some embodiments may also benefit from a nonlinearity correction.Although the dynamic range of the VAS channels accommodates energydeposition up to 1 MeV, the VAS channels may exhibit nonlinearity atenergies as low as 662 keV. Thus, an energy calibration using multipleenergy sources may be carried out. For multiple-pixel events, becausethe total deposited energy is derived from the sum of the depositedenergy by each individual interaction, an accurate energy calibrationmay facilitate good energy resolution.

Yet more benefits may result from a timing amplitude-walk correction, asdescribed above. Timing amplitude-walk may be present for both the anodeand cathode timing signals. To get a more accurate electron drift timefor multiple-pixel events over the whole energy range, a timingcalibration may be performed for all channels.

It should be noted that, in cases where the electron drift time is not alinear function of the depth of interaction, then the cathode-to-anodesignal ratio may be used as an indication of the depth of interaction.

Yet another correction factor involves a weighting potential cross-talkeffect between the interactions of a multiple-pixel event. Morespecifically, this correction step may correct for signal deficitsarising from cross-talk between interactions in multiple-pixel events,as described below.

For multiple-pixel events, the electron drift time may be used to getthe interaction depth z_(i). The deposited energy E_(i) may be firstestimated using equation

$E_{i}^{\prime} = \frac{Q_{i}\left( {x_{i},y_{i},z_{i}} \right)}{A\left( {x_{i},y_{i},z_{i}} \right)}$for each pixel. However, due to a cross-talk of the weighting-potential,each Q_(i)(x_(i),y_(i),z_(i)) is less than that corresponding to thetrue deposited energy E_(i). Generally speaking, the cross-talk mayarise from the induced effect of one electron cloud on the anode pixelassociated with another interaction. A weighting-potential cross-talkcalibration may be done to correct this signal deficit. The calibrationis generally based on the separation between the pixels involved in themultiple-pixel event as well as the interaction depth of the interactionconstituting the source of the deficit (i.e., via cross-talk with itselectron cloud). Alternatively, the correction factor data may be storedas a function of the interaction depths for both of the interactionsinvolved in the cross-talk. In other cases, the energy-weighted centroiddepth may be used to determine the correction factor for weightingpotential cross-talk (rather than one or both of the individualinteraction depths).

The cross-talk correction factor may also be based on the relativelateral positioning of the affected pixels. In one embodiment, thecorrection factor is a function of whether both affected pixels aredisposed on a periphery of the anode. See, for example, each of theanode pixels 68 depicted in FIG. 4. A non-peripheral anode pixel may beone disposed in an interior portion of the anode, e.g., the pixels foundin a 3×3 pixel square in the center of the anode. In some embodiments,the correction factor may also be a function of whether either of theinvolved pixels are in a corner of the anode. In any event, thecorrection factor data may then be stored for each of several possibleconfigurations of the multiple-pixel event. For example, differentcorrection factors may be applicable to otherwise identical events(i.e., same lateral distances, same interaction depth(s)) because oneevent has two peripheral interactions, and the other event has oneperipheral interaction and one interior interaction, two interiorinteractions, or any one of a number of other possibilities if aninteraction may fall into a non-peripheral, non-interior or non-cornercategory. Alternatively, the correction factor may only look to thelateral positioning of one of the interactions.

With reference now to FIG. 11, use of the cross-talk correction factorinvolves the detection of a multiple-pixel event in a block 98, afterwhich the electron drift time data is measured, determined and otherwiseprocessed in a block 100. Next, one or more first-order corrections maybe implemented in a block 102, such that, for instance, a C/A ratio isdetermined from the electron drift time and the pixel involved (see,e.g., FIG. 10). A block 104 may then access one of the databases or datasets to determine the correction factor associated with the C/A ratiodetermined by the block 102. From that factor, the interaction depth anda preliminary value of the energy may be calculated for each interactionin a block 106. Lastly, the preliminary energy value is adjusted via thecross-talk correction factor in a block 108, which, as described above,may be stored as a function of (and therefore determined by) one or moreof the interaction depths, the lateral distances between the affectedpixels, and the relative or absolute lateral positioning of one or moreof the affected pixels.

In one embodiment, the cross-talk correction factor may be determinedfor a multiple-pixel event having more than two interactions byaveraging the factors determined for each pair of interactions.Alternatively, the cross-talk correction factors may be summed orotherwise combined for each interaction. In these ways, the database ordata set of correction factors may avoid the challenge of storing datafor every possible combination of, for example, three- or four-pixelinteraction sets.

Generally speaking, the calibration of a radiation detector inaccordance with the disclosed techniques may use any source, asdescribed above, such as ¹³⁷ Cs 662 keV gamma-rays. The manner in whichsufficient data is generated and collected for both single-pixel eventsand multiple-pixel events is shown in connection with exemplaryembodiments in FIGS. 12 and 13.

For the generation of the depth correction factor based on the C/Aratio, single-pixel photopeak events of a known energy (such as 662 keV)in this calibration are used. Because each value of electron drift timecorresponds to a particular depth of interaction, and thus a particularvalue of C/A signal ratio, a plot of electron drift time versus C/Asignal ratio may be generated to obtain the relationship betweenelectron drift time and C/A ratio. As a result, the depth of interactioncorresponding to any C/A signal ratio may be determined from a measuredelectron drift time.

Referring to FIG. 12, raw spectrum data is collected for each anodepixel (after correction for the electronics response so that the signalis linearly proportional to the induced signal on the anode pixel) todevelop depth separated spectra. With each event that makes up thecollective spectrum, a single-pixel event is detected in a block 110 andthe C/A ratio and drift times are measured in a block 112.

The raw spectrum has a broadened photopeak because of the electrontrapping and weighting potential effect. For each pixel, everysingle-pixel event will be added into a depth spectrum according to itscathode to anode signal ratio (C/A ratio). Because each depth spectrumhas a very sharp, known photopeak, the broadened photopeak should becorrected to correspond to the known (e.g., 662 keV) energy deposition.Therefore, the energy correction factors are determined in a block 114by, for instance, measuring the centroids of these photopeaks anddetermining the correction factor to each depth spectrum needed tocombine them into the final spectrum so that all photopeaks will bealigned to the same location and form a sharp peak in the finalcorrected spectrum.

To wrap up the calibration procedure, the correlations between the C/Aratios and both the electron drift times and the energy correctionfactors are stored or recorded in any desired manner in blocks 116 and118, respectively.

Turning to FIG. 13, the calibration of a radiation detector to addressthe cross-talk between interactions of a multiple-pixel event beginswith the detection of the event in a block 120. The interaction depthsare then determined via, for instance, the procedure described above, ina block 122. In this embodiment, the lateral distance between the anodepixels and the relative lateral positioning of the pixels are thendetermined in blocks 124 and 126 to support the correction factordetermination. Using the known energy, and relying on the energycalculated for the interaction using any of the other calibrationsdescribed herein, the cross-talk correction factor is then determinedfor the given event data in a block 128. After storing the correctionfactor in association with the event data in a block 130, further datamay then be collected to cover different anode pixel combinations,different interaction depths, etc.

Two CZT detectors of identical dimensions were tested to evaluate theimplementation of one or more of the foregoing techniques. Gamma-rayevents from Cs 662 keV were collected and analyzed. The cathode biasvoltage was chosen for the stability of the cathode signal. The gridbias voltage was based on an electric field calculation to preventcharge sharing on the grid electrodes. The first detector was biased at2200 V on the cathode and 85 V on the anode grid. The second detectorwas biased at 2000 V on the cathode and 80 V on the anode grid. Theanode pixels were at ground potential and connected to the input of theASIC by wire-bonds. Both detectors were irradiated from the cathode sidewith gamma-ray sources placed 5 cm away from the cathode. The resultingCs 662 keV gamma-ray spectra shown in FIGS. 14A-14C were acquired over40 h for each detector. Spectra from Ba, Co and Na gamma-ray sourceswere also collected to support a nonlinearity calibration. After thecorrections based on the correlation between C/A ratio and electrondrift time (but not the cross-talk), the energy resolution (FWHM) forsingle-pixel and multiple-pixel events over the whole volume of eachdetector were as shown in Table 1 below.

TABLE 1 Single-Pixel Two-Pixel Three-Pixel Four-Pixel Detector EventsEvents Events Events 1 1.11% 1.57% 2.13% 2.64% @ −2200 (7.3 keV) (10.4keV) (14.1 keV) (17.5 keV) V, −85 V 2 1.14% 1.64% 2.28% 2.81% @ −2000(7.5 keV) (10.9 keV) (15.1 keV) (18.6 keV) V, −80 V

The overall spectra for single-pixel events, two-pixel events andthree-pixel events from the entire first detector are shown in FIGS.14A-14C. The rather complete absorption of the gamma-ray formultiple-pixel events is evident from the lack of background counts inFIGS. 14( b) and (c), which also demonstrates a beneficial capability ofthe gamma-ray spectroscopy enabled by the techniques disclosed herein,namely implementing event selection based on the signatures of theinteractions of a multiple-pixel event.

Two further exemplary CZT detectors were also tested, where electronicnoise was addressed via modifications to the front-end board. Themodifications to the connections configuration reduced cross-talk noiseon the cathode signal induced by digital control signals. Further,unlike the prior exemplary detectors, the detection scheme was notlimited by one global threshold for all the channels reading signalsfrom anode pixels. Rather, in these cases, the anode pixels hadtriggering thresholds ranging from 50 keV to 80 keV, with the spreadcaused by the variations of DC offsets and noise in each ASIC channel.To this end, a 4-bit digital-to-analog converter (DAC) unit was added toeach TAT channel so that the threshold of each channel could be finelytuned to achieve lower thresholds among the channels.

The first detector of this second exemplary pair was biased at −2200 Von the cathode and the second at −1400 V. The anode pixels wereDC-coupled to the ASIC inputs and thus all were at ground potential. Thecommon grid electrode between the pixels was biased at a negativevoltage to steer electrons drifting towards the anode pixels. The wholesystem was operated at room temperature (˜23 C.). The detector wasirradiated from the cathode side with non-collimated gamma-ray sourcesplaced 5 cm away from the cathode. Data collected from a 137 Csgamma-ray source was used for the calibration. Spectra from a 241 Amsource were also collected for measuring the electron mobility-lifetimeproducts and estimating the electronic noise.

With the help of the disclosed techniques, the material non-uniformity,the weighting potential variations and the electron trapping variationswere accommodated to the limit of the position resolution—estimated tobe 1.27 mm×1.27 mm×0.2 mm. By implementing the 3-D corrections,unprecedented energy resolutions of 0.93% and 0.76% FWHM at 662 keV forsingle-pixel events were achieved from the entire 2.25 cm³ volumes ofthe detectors. As a result of the much lower thresholds than theprevious systems, the 32 keV 137 Cs K x-rays were also observed in bothsystems. For multiple-pixel events, the depth of each interaction wasderived from the electron drift time for each pixel. After correctionfor timing-amplitude-walk, electron trapping and non-linearity for eachsignal, the true energy and 3-D position information were obtained foreach interaction. An energy resolution of 1.23% FWHM at 662 keV wasachieved for two-pixel events collected from the entire volume of thesecond detector. The energy resolutions (FWHM) of 662 keV single-pixeland multiple-pixel events are summarized in Table II for the twodetectors.

TABLE 2 Single-Pixel Two-Pixel Three-Pixel Detector Events Events Events1 0.95% 1.52% 2.67% @ −1400 (6.29 keV) (10.06 keV)  (17.68 keV) V, −45 V1 0.93% 1.46% 2.41% @ −2200 (6.16 keV) (9.67 keV) (15.95 keV) V, −65 V 20.76% 1.23% 2.3%  @ −1400  (7.3 keV) (8.14 keV) (15.22 keV) V, −45 V 20.78% 1.19% 2.08% @ −2200 (5.16 keV) (7.88 keV) (13.77 keV) V, −60 V

Embodiments of the disclosed system and method may be implemented in orinvolve hardware, firmware or software, or any combination thereof. Someembodiments may include or involve computer programs executing onprogrammable systems comprising at least one processor, a data storagesystem (including volatile and non-volatile memory and/or storageelements), at least one input device, and at least one output device.Program code may be applied to input data to perform the functionsdescribed herein and generate output information. The output informationmay be applied to one or more output devices, in known fashion. Forpurposes of this application, a processing system includes any systemthat has a processor, such as, for example, a digital signal processor(DSP), a microcontroller, an application specific integrated circuit(ASIC), or a microprocessor.

The programs may be implemented in any high level procedural or objectoriented programming language to communicate with a processing system.The programs may also be implemented in assembly or machine language, ifdesired. In fact, practice of the disclosed system and method is notlimited to any particular programming language. In any case, thelanguage may be a compiled or interpreted language.

The programs may be stored on a storage media or device (e.g., floppydisk drive, read only memory (ROM), CD-ROM device, flash memory device,digital versatile disk (DVD), or other storage device) readable by ageneral or special purpose programmable processing system, forconfiguring and operating the processing system when the storage mediaor device is read by the processing system to perform the proceduresdescribed herein. Embodiments of the disclosed system and method mayalso be considered to be implemented as a machine-readable storagemedium, configured for use with a processing system, where the storagemedium so configured causes the processing system to operate in aspecific and predefined manner to perform the functions describedherein.

While the present invention has been described with reference tospecific examples, which are intended to be illustrative only and not tobe limiting of the invention, it will be apparent to those of ordinaryskill in the art that changes, additions and/or deletions may be made tothe disclosed embodiments without departing from the spirit and scope ofthe invention.

The foregoing description is given for clearness of understanding only,and no unnecessary limitations should be understood therefrom, asmodifications within the scope of the invention may be apparent to thosehaving ordinary skill in the art.

1. A radiation detector for detecting a multiple-pixel event having aplurality of radiation interactions, the radiation detector comprising:a first data processing module that generates electron drift time datafrom anode and cathode signals arising from the plurality of radiationinteractions; and, a second data processing module that accesses a firstdata correlation of electron drift times with cathode-to-anode signalratios and a second data correlation of cathode-to-anode signal ratioswith correction factor values for a radiation characteristic for themultiple-pixel event; wherein the second data processing moduledetermines the radiation characteristic based on the electron drift timedata, the first data correlation and the second data correlation.
 2. Theradiation detector of claim 1, wherein the second data processing modulefurther determines an interaction depth for a selected radiationinteraction of the plurality of radiation interactions based on theelectron drift time data and the first data correlation.
 3. Theradiation detector of claim 2, wherein the second data processing systemfurther determines a distance between a first pixel of the radiationdetector associated with the selected radiation interaction and a secondpixel with which the radiation also interacts in the multiple-pixelevent.
 4. The radiation detector of claim 3, wherein the second dataprocessing system further accesses a third data correlation to determinea further correction factor for the radiation characteristic based onthe interaction depth and the distance between the first and secondpixels.
 5. The radiation detector of claim 4, wherein the third datacorrelation specifies values for the further correction factor based onlateral positioning of the first and second pixels.
 6. The radiationdetector of claim 5, wherein the values for the further correctionfactor vary based on whether one or both of the first and second pixelsare disposed along an anode periphery or within a central anode area.